FK506 is a natural product which binds to an FK506-binding protein, FKBP, with high affinity to form an FK506:FKBP complex. Reported Kd values for that interaction are as low as 400 pM. The FK506:FKBP complex binds with high affinity to the protein phosphatase calcineurin to form a tripartite, [FKBP:FK506]:[calcineurin], complex. Calcineurin is a heterodimer of a catalytic subunit (calcineurin A) and a regulatory subunit (calcineurin B.) In this tripartite complex FK506 acts as a dimerizer or adapter to join FKBP to calcineurin.
Numerous naturally occurring FK506 binding proteins (FKBPs) are known. See e.g. Kay, 1996, Biochem. J. 314:361–385 (review). FKBP proteins have been used for their ligand-binding properties in biological switches based on ligand-mediated multimerization of immunophilin-based recombinant proteins as disclosed e.g. in Spencer et al, 1993, Science 262:1019–1024 and in WO 94/18317.
Cyclosporin A is another macrocyclic natural product of interest. It binds to the protein cyclophilin to form a complex which also binds to calcineurin to form an immunosuppressive complex. Cyclosporin thus also acts as a dimerizer. While the potent immunosuppressive activity of FK506 and cyclosporin would limit their utility as a dimerizer, especially in animals, this invention harnesses their dimerizing potential (and that of related compounds) while avoiding their profound, inherent limitations.